Catalytic Routes for Direct Methane Conversion to Hydrocarbons and Hydrogen: Current State and Opportunities

Abstract

Natural gas, the cleanest fossil fuel, is an abundant source of methane and expected to play an increasingly important role in powering the world’s economic growth over the energy transition of the coming decades. Methane has the potential to be a CO2-free feedstock to cogenerate hydrogen (H2) and added value “building-blocks” chemicals (e.g., olefins and aromatics) for petrochemistry. In this review, the two processes (i) the oxidative coupling of methane (OCM) for production of ethylene and (ii) the nonoxidative methane dehydroaromatization (MDA) producing hydrogen and benzene are discussed. Both routes convert methane directly into valuable products, an advantage over the several-steps syngas route. The performances of various a variety of catalysts reported during the last 25 years for OCM (MnNaW, La2O3, Li-MgO, etc.) and MDA (M/HZSM-5, M/TNU-9, M/IM-5, M/ITQ-2, M@SiO2, M@CeO2, TaH/SiO2, GaN/SBA15, single-site M@HZSM-5, bimetallic M-M′/HZSM-5, core–shell structures, M/Zr­(SO4)2 with M = Mo, Fe, Pt) under similar reaction conditions are compared. The major drawbacks and the strategies used to mitigate the main challenges related with the performance of the catalysts in both OCM and MDA reactions are critically revealed. For instance, the overoxidation in the OCM is mitigated by optimizing of the operating conditions, using alternative oxidants, and the application of membrane reactor technology are discussed. In the MDA reaction, the major issue is the catalyst deactivation by coke formation and migration and sintering of metallic active phases. Strategies for robust catalysts, methods for mild coke removal, pretreatment under reductive atmosphere are presented. Approaches to improve aromatics yields over coke production by addition of promoters or co-feed reactants to the MDA catalysts are also discussed.

Coke relocation and Mo immobilization in donut-shaped Mo/HZSM-5 catalysts for methane dehydroaromatization

“…The methane consumption, k d , and combustion temperatures of so coke and hard coke, and the remaining microporous volume of the parent [Mo]-Z P catalyst are similar to the ones reported for ZSM-5 zeolite earlier. 6,9,38,42 Thus, the parent sample [Mo]-Z P can be considered as a reliable reference in order to understand the behavior of the donut-shaped zeolite catalysts in the MDA reaction. considered as a precursor for MDA active sites, 9 thus lowering the methane consumption.…”

“…4,5 Catalyst development is one of the strategies identied to address such challenges. 3,6 Different ways of catalyst improvement have been reported, such as Mo/Al balance tuning, [7][8][9] variation of the molybdenum loading approach, 10,11 pretreatment, [12][13][14] metallic promoter addition, [15][16][17] or crystal engineering. [18][19][20][21] The latter represents an extensive research topic for hydrocarbon processing catalyzed by acidic-microporous materials since the diffusional limits in zeolites lead to coke accumulation and mitigate the efficiency of the crystal core.…”

C-Vacancy Mediated Methane Activation and C–C Coupling on TiC(001) Surfaces: A First-Principles Investigation

“…There are two common routes for C−C coupling� oxidative coupling of methane (OCM) and nonoxidative coupling of methane (NOCM). 28 OCM has some drawbacks such as the overoxidation of the C 2 products to form CO 2 and low selectivity. 29 NOCM is preferred over OCM since no oxidizing agents are required, and H 2 , which is an useful fuel, can be obtained as one of the side products.…”

“…Among the various valuable chemicals that could be obtained from CH 4 are the C 2 products characterized by C–C bonds. There are two common routes for C–C couplingoxidative coupling of methane (OCM) and nonoxidative coupling of methane (NOCM) . OCM has some drawbacks such as the overoxidation of the C 2 products to form CO 2 and low selectivity .…”

Mechanistic Insight into the Direct Nonoxidative Conversion of Methane to Ethylene over Structure-Sensitive RhO2/TiO2 Catalysts

“…The high C–H bond energy (104 kcal·mol –1 ) and the intrinsic nonpolar nature of CH 4 make the direct activation of its stable C–H bond tremendously challenging during catalytic conversion . Among the various catalysts applied for the direct conversion of methane, supported metal oxide catalysts for the direct conversion of methane to high-value-added target products, such as olefins, have been well-developed. , Lunsford investigated the reaction mechanism of the methane oxidation coupling process over Li/MgO catalysts and proposed that, on the one hand, the catalyst should be able to activate methane efficiently to produce methyl radicals and, on the other hand, it should avoid deep oxidation of methane to byproducts such as carbon monoxide and carbon dioxide. , Thermodynamically, deep oxidation of methane is favorable, but it is difficult to ensure a high conversion rate and high selectivity of methane oxidation coupling. However, nonoxidative conversion of methane to ethylene can achieve high atomic utilization while avoiding byproducts.…”